When a printing stop factor such as a toner shortage or a failure in an optical system occurs, printing is immediately stopped (e.g., see Japanese Patent Laid-Open No. 60-48658). Some color printers which use cartridges of a plurality of colors to print an image check the tone of each color and change the toner density. If a printing stop factor such as a toner shortage occurs in one of cartridges of a plurality of colors, printing is stopped even if no printing stop factor occurs in the remaining color cartridges (e.g., see Japanese Patent Laid-Open No. 60-73645).
As described above, in a conventional color printer which uses cartridges of a plurality of colors, if a printing stop factor occurs in at least one of the color cartridges, printing is stopped even if no problem such as a toner shortage occurs in the remaining color cartridges. For this reason, for example, even a printout in a single color cannot be produced using a cartridge of another color which has no problem such as a toner shortage.
To solve this problem, there is proposed an apparatus which can perform image forming in a single color using a monochrome cartridge even when, for example, a toner shortage occurs in any of color cartridges in a color printer (see, e.g., Japanese Patent Laid-Open No. 2001-83750).
However, if a toner shortage occurs in any of color cartridges in a color printer, and printing is continued using a monochrome cartridge for a long time, a problem occurs. More specifically, a cleaning blade for cleaning the image carrier of the color cartridge having run short of toner may be turned up, and an image on a printing paper sheet being conveyed may degrade.